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Introduction

Virtually every polymer can be used in film form. Most thermoplastic polymer
films are prepared by conventional extrusion techniques based on calendering,
casting, and blown-film, or tenter-film systems. Other polymers, which cannot be
easily melted, are formed into films by solvent casting. In the selection of a film for
a particular application, the properties of the polymeric materials must be con-
sidered in view of the application. Thermal properties, molecular characteristics,
and crystallinity of the polymer affect processing and film properties. Additives
influence extrusion and orientation processes and improve film properties.

Polymer Properties

Molecular Weight and Molecular Weight Distribution.

Molecular

weight and molecular weight distribution affect extrusion and orientation pro-
cesses and the physical properties of the film, eg, tensile strength, elongation,
impact resistance, (qv) and optics (see M

OLECULAR

W

EIGHT

D

ETERMINATION

).

Films prepared from high molecular weight materials exhibit excellent me-

chanical strength, impact strength, and orientability. As molecular weight in-
creases, melt strength increases. Tubular orientation requires high molecular
weight and a melt index (MI) of 0.1–3 (ASTM D1238E) to provide adequate melt
strength during extrusion and orientation. However, high molecular weight im-
pedes processing and results in a film with poor optical clarity, despite orienta-
tion. Films of lower molecular weight material are brittle, difficult to orient and
have lower melt strength. Low molecular weight resins are characterized by good
clarity and high extrusion or processing rates. Resins of low molecular weight

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

284

FILMS, MANUFACTURE

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with a melt flow (MF) of 6–12 (ASTM D1238E) are used for sheet casting because
melt strength is not required but good clarity is of prime importance. For tenter-
process extrusion, polymers of a middle range are employed, eg, as polypropylene
(PP) with MF 1.5–7 (ASTM D1238L), because melt strength is less critical. This is
also true of tubular film processes that employ an initial melt quenching step. At
the same molecular weight, optics, draw down, and impact strength increase as
molecular weight distribution narrows, whereas melt strength, die swell, shrink-
age, and processibility decrease.

Catalyst systems used for polymerization have a large impact on the molecu-

lar weight distribution and have undergone tremendous changes in the last decade
(1). As older catalysts systems are replaced by new catalysts, film manufacturing
processes and material selection have become more critical to ensure maintenance
or improvement in film properties without adversely impacting the manufacturing
process.

Crystallinity.

The properties of a material are determined by its molec-

ular structure and bulk properties are controlled by the spatial arrangement of
the molecules. Polycrystalline materials are best understood in terms of a two-
phase model consisting of amorphous and crystalline domains (2). The properties
of each phase differ (see A

MORPHOUS

P

OLYMERS

; S

EMICRYSTALLINE

P

OLYMERS

). The

observed properties of a material depend on the component-phase properties and
the interaction of the two phases. Structural parameters are determined by the
ratio of the two phases and the average orientation of the molecules in each phase
(3). For polyolefin materials, such as polypropylene and polyethylene, new cat-
alyst systems based upon metallocene materials are dramatically changing the
molecular structure of the polymer chain relative to existing catalysts. The new
molecular structure results in new material properties because of better control
of the polymerization process. These new materials exhibit differences in melt-
ing point, crystallinity, and molecular weight distribution, and have a dramatic
impact on film processing conditions and final film properties (4).

Increasing crystallinity of a semicrystalline polymer is accompanied by in-

creases in modulus, stiffness, density, yield stress, chemical resistance, melting
point, glass-transition temperature, abrasion resistance, creep resistance, and di-
mensional stability, and by reduction in impact resistance, elongation, thermal
expansion, permeability, and swelling.

Final film crystallinity and crystalline- and amorphous-phase orientation for

a given polymer depend strongly upon film processing conditions.

Thermal Properties.

The chemical nature of the polymer determines its

stability to temperature, light, water, and solvents. Polymer chains form ordered
structures, and the thermodynamics of this ordered state determines melting
point T

m

, the glass-transition temperature T

g

, and mechanical and electrical

properties. Thermal properties (qv) of film polymers are given in Table 1 (5–11).
The polymers best suited for films have a glass-transition temperature below
0

◦

C, a melting point above 100

◦

C, and a decomposition temperature at least 50

◦

C

above the melting point. Such polymers, eg, polyethylene (PE), polypropylene (PP),
and polystyrene (PS), can be extruded in conventional extruders. Semicrystalline
polymers with T

g

above room temperature, such as poly(ethylene terephthalate)

(PET), are most suitable for films if oriented to improve room temperature physical
properties but are prone to pin hole cracking if flexed excessively.

Table 1. Thermal Properties of Film Polymers

a

Glass-transition

Melting

Decomposition

Common

temperature

temperature

temperature

Class

Polymer

symbol

T

g

,

◦

C

T

m

,

◦

C

T

dec

,

◦

C

Polyolefin

High density polyethylene

HDPE

−120 to −140

130–137

335–450

Low density polyethylene

LDPE

−95 to −130

106–110

335–450

Polypropylene

PP

−10

170

328–410

Ethylene–propylene copolymer

EP

−30

128–145

328–410

Poly(1-butene)

PB

−24

126

288–425

Poly(4-methyl-1-pentene)

TPX

30

235

335

Polytetrafluoroethylene

PTFE

27

330

508–538

Vinyl polymers

polystyrene

PS

85–105

240

285–440

Poly(vinyl acetate)

PVAC

75–105

240

213–325

Poly(vinyl chloride)

PVC

75–105

212–310

200–300

Poly(vinylidene chloride)

PVDC

−18

210

225–275

Poly(vinyl fluoride)

PVF

40

200–235

372–480

Polyacrylonitrile

PAN

97

320

250–280

Poly(methyl methacrylate)

PMMA

65

160

170–300

Polyethers

Polyoxymethylene

POM

−85 to −110

181

100

Poly(phenylene sulfone)

220

300

310

Cellulose triacetate

LA

105

306

250–310

Polyesters

poly(ethylene terephthalate)

PET

81

265

283–306

Polycarbonate

PC

147

225

420–620

Polyamides

Polycaprolactam

NYLON-6

40–87

220

310–380

Polyhexamethyleneadipimide

NYLON-6,6

50

265

310–380

Polyundecanoamide

NYLON-11

46

194

310–380

Polyimide

310–365

310–380

a

Temperature quoted represents values for specific grades of commercial materials and may not be representative of all materials of this classification.

285

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When the polymers’ melting temperature is too close to the decomposition

temperature a film can be produced by solvent casting. Alternatively, a reduction
of the melting point of the polymer by copolymerization (12) is possible. This re-
duces the crystalline order of the polymer and lowers the melting point by decreas-
ing the energy required for melting. In addition, the decomposition rate can be
controlled by adding stabilizers. Extruder and die are designed with streamlined
flow lines to avoid melt stagnation and push the polymer through extruder and
die as quickly as possible. A lubricating additive may also be added to minimize
melt adhering to the metal surface, reducing residence time and degradation. Un-
plasticized poly(vinyl chloride) (PVC), polyamides, and polyesters are extruded in
this way (13). When the melting temperature is above the decomposition temper-
ature, as with cellulose, poly(vinyl alcohol), or polyacrylonitrile, solvent casting
and coagulation are used.

When no solvent is convenient and the use temperature of the film is be-

low the T

g

of the polymer, a nonvolatile liquid is required, which acts as solvent

at high temperatures and as plasticizer at room temperature; both the T

g

and

the processing temperature are reduced. PVC plasticized with phthalates up to
half its weight (13) can be extruded, as can be vinylidene chloride–vinyl chloride
copolymer.

Additives.

Organic and inorganic additives or coatings impart function-

ality to the finished product as demanded by the customer (9,14–16) (see
Table 2). Additives (qv) must conform to government regulations for material
handling and health hazards for both workers and consumers.

Stabilizers reduce degradation during film processing to prevent formation

of lower molecular weight polymers, which would result in poor film properties,
gels, and discoloration. Hindered phenol antioxidants combined with phosphites
reduce thermal degradation of polyolefins. Other stabilizers improve long-term
stability to heat and light (see S

TABILIZATION

).

Slip agents such as saturated and unsaturated fatty acid amides can over-

come film surface friction. These agents migrate to the film surface during pro-
cessing and upon aging at elevated temperatures (see R

ELEASE

A

GENTS

).

Antiblock agents (qv) are typically used to prevent adherence, ie blocking, of

two film surfaces. They are small particle-size fillers affecting surface smoothness,
controlling film-to-film surface separation distances without greatly affecting
clarity.

Antistatic agents (qv), typically amine derivatives of glycerol monostearate,

are primarily used to reduce static generation in packaging equipment. These
additives are particularly effective for films designed for packaging powders by
minimizing dust pick up on seal areas and package surfaces.

Rheology.

Rheology is the study of deformation and flow. For polymers the

viscous and elastic behavior of melts determines the design of extruder screws,
melt pipes, filters, and die configuration. The elastic nature of the melt at any tem-
perature affects surface smoothness, layer thickness, and stability. Solid phase
rheology influences stretching, such as that occurs in the stepwise or simultane-
ous biaxial orientation. The rheology of the finished product also determines film
behavior on packaging machines, printing presses, or laminators where the film
is both heated and loaded by tension changes (14,15) (see also R

HEOLOGY

).

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287

Table 2. Film Additives

Additive function

Supplier

Trade name

Chemical name or class

Antioxidants

Goodrich

Goodrite 3114

Phenolic

Goodrich

Goodrite 3140

Amine

Uniroyal

BHT

Butylated hydroxytoluene

Albright & Wilson

Albrite DLHP

Organic phosphite

AKZO Nobel

Tributyl phosphite

GE Specialty

Dilauryl phosphite

Chemicals

CIBA-GEIGY

Irgonox

High molecular weight

hindered phenols

Eastman

Poly TDP-2000

Thiodipropionate polyester

Antistatic agents

ICI Surfactants

Atmer

compound

compound

Rhone-Poulenc

Rhodafac RE-610

Anionic

AKZO Nobel

Armostat

Amines

ICI Surfactants

Atmer 122, 122K

Glycerol esters

Heat stabilizers

Cardinal

CC-7711

organotin

American

Cyastab

Lead-based carbonates,

Cyanamide

sulfates, phthalates

Ciba Specialty

Irganox B-Blends

Phosphites

Chemicals

Ferro

Synpron

Barium–cadmium powder

Total Specialty

Cal-Z Series

Cadmiun–zinc liquids

FERRO

Snypron

Antomony mercaptides

Uniroyl Chemical

Naugard

Amines

UV stabilizers

Cytec

Cyasorb

Benzophenone

Ciba Specialty

Tinuvin

Benzotriazole

Chemicals

Clariant

Sanduvor

Oxalanide

Ciba Specialty

Tinuvin

Hindered amine

Chemicals

(HALS)

a

Slip agents

Witco

Kenamide

Fatty acid amide

Dow Corning

Dow 200 fluid

Polydimethylsiloxane

Allied

A-C Polyethylene

PE wax

Admer

Polyolefin

Fillers, antiblock

Georgia Marble Co., OMYA Inc.

Calcium carbonate

agents

DiCaperl Hp-510

Glass bubble

North Georgia Minerals & Chemicals

Aluminum trihydrate
Celstite
Barites
Talc

James River

Solka-Floc

Cellulose

Manville

Celite

Diatomaceous earth

Manville

Micro-Cel

Hydrous calcium silicate

Suzorite

Mica

Wollastonite

Calcium-m-silicate

Glomax

Calcined kaolin clay

Kaopaque

Delaminated aluminum

silicate

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FILMS, MANUFACTURE

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Table 2. (Continued)

Additive function

Supplier

Trade name

Chemical name or class

3M

Zeeosphere

Ceramic spheres

Antimicrobial

Morton

Vinylzene

10,10-Oxybisphenoxarsine-2-n

Ferro

Micro-Check

2-n-Octyl-4-isothiazolin-3-one

Olin Corp.

Omacide

Zinc pyrithione

a

HALS

= hindered amine light stabilizer.

Extrusion-Based Manufacturing Processes

Extrusion.

Extrusion (qv) is widely used for the manufacture of polymeric

films because it permits the preparation of highly uniform polymer melts at high
rates (17–20). An extruder (Fig. 1) consists of a hollow cylindrical barrel fitted with
external heaters. The inner surface is coated with a hard metal liner such as Xaloy.
A screw is fitted into this cylinder with a specific geometry determined by the
polymer and the desired thermal condition of the melt. The screw is driven by an
electric motor through a gear reducer sized for the speed and power requirements
of the screw. The typical barrier screw shown in Figure 2 takes advantage of
the melting mechanism in the extruder to increase efficiency. Most of the energy
required to melt the polymer is supplied by the motor (21,22). Barrel temperature
is maintained by electric heaters, which often contain channels for cooling water.

Fig. 1.

Extruder.

Fig. 2.

Barrier flight screw.

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FILMS, MANUFACTURE

289

The solid polymer is introduced into the feed throat of the extruder and

conveyed into the screw by its turning motion (17). It is compacted into a solid plug,
which is melted by contact with the hot barrel. Molten polymer is collected by the
screw flights and pumped toward the end of the extruder. At the end of the screw,
the polymer is completely melted. The melt is mixed by the screw rotation, which
generates enough pressure to push the melt through a die and to the next step
(23,24). The condition or quality of the melt is extremely important for film quality
and process stability. The extrusion conditions affect crystallization behavior and
molecular weight of the film.

The melt is produced by the viscous dissipation of mechanical energy into

a very thin layer of molten polymer located between the compressed solid plug
and the heated barrel surface (18) (Fig. 3). Owing to the high viscosity of the
polymer melt, typically 50–1000 Pa

·s (500–10,000 P), large amounts of energy

are dissipated in the melt as the motor turns the screw. The energy is converted
into heat, which is transferred to the solid polymer and the barrel, raising the
temperature of the melt. Excessive temperatures or mechanical work can cause
thermal, oxidative, or mechanical degradation of the polymer. A new generation
of energy-efficient screw designs have been developed, which minimize the melt
temperature by mixing the collected melt with the solid particles of the compacted
bed (25,26). This is accomplished by purposely disrupting the melting mechanism
of Figure 3 at some point, by changing the screw geometry to mix the solid with
the melt.

The polymer melt flows through a die to the next process step; flat, circular,

or slot dies are preferred. The flow through the die must be uniform across the exit
plane. However, this is complicated by the nonlinear dependence of melt viscosity
on both temperature and shear rate in the die (23,24). The suitability of a material
is determined by measuring the flow properties with a capillary rheometer in the
temperature and shear-rate range expected. Melt elasticity can cause flow insta-
bilities, which affect haze and thickness (27,28) or the operation of downstream
equipment. Exit melt velocity, flow characteristics, and quenching rate may im-
part significant orientation to the polymer. In some instances, melt orientation is
reduced; in others, it is maintained by quenching.

In flat-die extruders, slot and coat hanger die geometries (Fig. 4) predom-

inate, whereas in circular die extruders, spiral dies (Fig. 5) predominate. A die

Fig. 3.

Melting mechanism of single screw extruders.

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FILMS, MANUFACTURE

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Fig. 4.

Coat-hanger die.

Fig. 5.

(a) Tubular (three-layer) spiral die; (b) detail of spiral melt manifold.

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FILMS, MANUFACTURE

291

is usually constructed of tool steel and heated to maintain the desired tempera-
ture for optimum flow. Temperature uniformity is very important. A uniform flow
through the die is determined by the design of the internal distribution manifold.
For good melt distribution, this section must be designed to maintain uniform
flow rate, pressure drop, and shear (24). Independent flow adjustment is usually
provided by an adjustable die lip or choker bar. These devices ensure a uniform
flow by selectively changing the shear rate of the melt before it leaves the die.
Multiple temperature zones, standard on most dies, permit the selective alter-
ation of the temperature across the die face, ensuring production of a flat sheet.
This is necessary to counteract gauge variations caused by possible nonhomoge-
neous melt from the extruder, inadequacies in the die design, and the physical
limitations imposed in the machining of the internal configuration of the dies.

To ensure isothermal flow, die temperatures are maintained as close to the

melt temperature as is practical. Die gaps vary, depending upon the speed with
which material is removed from the die lips, as this determines the shear rate.
This is important because of die swell and melt fracture. Die gap also determines
the final film thickness. Die gaps are 0.25–2.5 mm (0.010–0.10 in.), which is small
compared with the typical die width of 39–305 cm.

Coextrusion.

Coextrusion technology has been developed in conjunction

with new polymers, providing the film structures required for flexible packaging
(see C

OEXTRUSION

). Coextrusion is perhaps the most economical method of com-

bining polymers into functional multilayer films.

Multiple melt streams from several extruders are combined either in a feed

block (Fig. 6) or in a multiple cavity die (Fig. 7), resulting in a stratified flow from

Fig. 6.

Coextrusion system (single cavity die) using a feed block for three polymer streams.

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FILMS, MANUFACTURE

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Cavity B

Cavity C

Cavity A

Fig. 7.

Multiple cavity coextrusion die with multiple melt-distribution manifolds.

the die. A feed block may be used with a single cavity die or in conjunction with one
cavity of a multiple cavity die to produce structures with many layers. Multiple
cavity dies are used for films requiring several layers, which interact chemically
to improve adhesion between the polymers but which may disturb the interface
stability or noninteracting polymers, which must remain separate for as long as
possible owing to poor rheological compatibility.

Coextrusion is promoted by the laminar flow of the melt in the feed block

and die, which prevents the mixing of the various layers. The laminar flow is
due to the low Reynolds numbers (low inertia between flow planes) that result
from the high melt viscosities (23,24). However, the generation of interfaces be-
tween the flowing materials requires that melt viscosities and melt elasticity
between the layers be sufficiently matched to prevent the formation of flow insta-
bilities (29–32). Therefore, coextrusion requires superior equipment and process
control throughout.

Multilayer spiral dies are becoming more common but are limited to rheo-

logical compatible polymers in the layers much as with the feed block method in
flat dies. This is due to the long flow distances required for layer uniformity in
the multilayer spiral dies, which makes them more prone to generate interfacial
instabilities.

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FILMS, MANUFACTURE

293

Fig. 8.

Water bath–chill roll quenching system.

In another method, two or three melt streams from separate extruders can

be combined and cast to form a multilayer structure. Alternatively, a melt stream
and a quenched web or a quenched and uniaxially oriented web are combined on
a chilled drum, providing the web with new surface functionality. This method is
known as the extrusion coating process (14).

Casting.

In casting, a self-supporting film or sheet is formed from a melt

supplied by an extrusion system. Processing conditions have a profound effect
on sheet properties and subsequent operations. Depending on the polymer, the
rate of heat removal determines the extent and morphology of crystallinity of the
sheet and the degree of residual orientation. Also important is the temperature at
which the heat is removed from the sheet. Thus, heat-transfer analysis of casting
processes is essential for the design of an efficient system.

Quenching is a continuous operation and must be regarded as an unsteady

state. It can take place in water, on a continuous steel band, on a casting drum,
or by a combination of these methods (see Fig. 8). The heat-transfer analyses for
these systems are similar, differing primarily in the magnitude of heat-transfer
coefficients. Heat-transfer rate, heat-transfer temperature, melt temperature, and
crystallization kinetics determines the optimum quenching conditions. The cast
sheet must be strong enough to be transported to the next stage of the manufac-
turing process.

If a high quality surface or a uniform heat-transfer rate is required, a pinning

step may be needed. Pinning is defined as the forcible application of the molten
film to the casting surface, it may be carried out by an air knife, a vacuum box, a
nip roll, or a strong electrostatic field. Pinning is needed for high production rates
while maintaining uniform properties across the quenched web. The pinning force
prevents air from being trapped between the film and the casting surface. Choice
of pinning method depends upon the speed of the operation and the quality of
the sheet required for the particular product to ensure intimate contact with the
surface against which the polymer is being solidified.

Examples of cast film are high clarity, low density polyethylene (LDPE) film

for bread bags, pallet wraps, PVC sheets, and polycarbonate (PC) or acrylic sheets
for glazing applications. Casting is the initial step in the production of biaxially
oriented PP, PS, and PET films.

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FILMS, MANUFACTURE

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Uniaxial Orientation.

Uniaxial orientation enhances the physical proper-

ties of sheets and films in one direction; properties in the perpendicular direction
are usually far inferior (29). The orientation temperature, governed by the mate-
rial, and the extent of orientation determines the final properties. Semicrystalline
polymers (qv) are oriented between the glass-transition temperature T

g

and the

melting point T

m

, whereas amorphous polymers (qv) are oriented above T

g

. In

general, rapid stretching in the temperature ranges described above, followed by
rapid quenching, orients the polymer. Rapid quenching ensures that the orienta-
tion is not lost by molecular relaxation. The degree of orientation is determined
by the extension rate and the degree of extension (3). A wide variety of com-
mercial equipment is available for uniaxial orientation, as well as proprietary
equipment developed for special processes or high production rates (see F

ILMS

,

O

RIENTATION

).

Orientation is a continuous operation and occurs in the direction of the

film motion or the machine direction (MD). Usually, a cast sheet is transported
on heated rollers (Fig. 9) to permit the sheet to reach a uniform temperature
at which the polymer molecules are sufficiently mobile. Having reached this
temperature, the sheet is abruptly accelerated between two rollers of different
speeds. This point is defined as the draw point. In some instances, additional
heat may be required to boost the temperature, or nip rolls are needed to reg-
ulate tension and to prevent film slippage (Fig. 9). Uniaxial film can be ob-
tained by orienting perpendicular (transverse) to the MD using tenter frames
or melt inflation, where the melt removal rate is equal to the melt velocity at the
die. However, these two methods are seldom employed. The properties required
by the user determine the orientation required to develop the desired physical
properties.

Fig. 9.

Machine-direction (MD) orientation.

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FILMS, MANUFACTURE

295

Amorphous and semicrystalline polymers behave very differently when sub-

jected to uniaxial orientation. Amorphous polymers may be drawn to any extent
or thickness if properly supported, whereas semicrystalline polymers show dis-
tinct and abrupt thickness changes, called necking, when drawn (3). Necking may
limit the rate at which the polymer is drawn because of the heat generated in
the neck region. The heat generated can be high enough to cause chain scission,
resulting in breakage of the sheet at the draw point (33). Necking serves to de-
fine a minimum draw ratio for the material at the temperature and draw rate
employed.

The relationships between physical properties and molecular orientation

of uniaxial products are well understood for both amorphous and semicrys-
talline polymers (34). The principal change that occurs upon orientation
is the preferential alignment of the c-axis of molecular chains in the direc-
tion of the orientation in the amorphous and crystalline phases of semicrys-
talline polymers (35,36). The preferred c-axis orientation allows the strong
covalent bonds along the chain backbone to carry loads applied in the orien-
tation direction. Perpendicular to the orientation direction, the weaker inter-
molecular van der Waals forces predominate between the molecular chains,
giving a much lower load-carrying capability. For amorphous polymers, an av-
erage orientation function defines the only measure required to define the
orientation state completely; it relates directly to the load-carrying amor-
phous phase. In semicrystalline polymers, the Hermans orientation func-
tion (3) describes the orientation contribution by the crystalline phase. This
must be combined with the amorphous phase orientation to describe an av-
erage molecular orientation. Average orientation is more difficult to relate
directly to physical properties because the crystalline phase does not con-
tribute greatly to the load-bearing capabilities of the structure. Therefore, a
more detailed analysis of the semicrystalline materials is required to deter-
mine the contributions from the amorphous and crystalline phases (34). Ad-
vanced analytical techniques are needed that are not readily available to many
workers.

Biaxial Orientation.

Biaxial orientation substantially improves the phys-

ical properties of the film and increases its commercial value. Biaxially oriented
films are produced by stretching polymers in directions perpendicular to each
other in the melt or rubbery state. The improvement over uniaxially oriented films
is due to the redistribution of c-axis chain orientation (36). This results in a film
with enhanced physical properties in both directions. The direction perpendicular
to the MD is defined as the transverse direction (TD).

Commercial biaxially oriented films are produced by the tenter-frame

(Fig. 10), double-bubble (Fig. 11), or blown-film process (Fig. 12). In the tenter-
frame and double-bubble processes, the polymer is stretched in the solid
state below the crystalline melting point. In the blown-film process, the poly-
mer is oriented in the melt, followed by rapid quenching to immobilize the
orientation.

The theoretical descriptions of biaxial orientation and the relationships to

physical properties are not as well developed as those of uniaxial orientation.
In amorphous polymers, birefringence can measure total molecular orientation
and can be used to predict properties. For semicrystalline polymers, a more

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FILMS, MANUFACTURE

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Fig. 10.

Tenter orientation.

detailed description of the orientation is required before prediction of properties is
possible.

Average-orientation, amorphous-orientation, and crystalline-orientation

functions are required to describe the oriented state completely (3,36). This de-
scription permits the calculation of amorphous-phase orientation, which is re-
quired to relate and predict physical properties. The determination of crystalline
orientation requires the more advanced analytical techniques of X-ray diffraction
or dichroic ratios from polarized infrared spectroscopy. Average or total orientation
functions are measured by birefringence. Amorphous and crystalline orientation
functions are separated by measuring the sonic modulus and assuming a molec-
ular model for the semicrystalline polymer (3).

Tenter-Frame Process.

Tenter frames (Fig. 10) consist of two side-by-side

endless chains that diverge at constant angle. The divergence of the chains forces
the polymer to stretch as it is transported along the chain, and imparts the de-
sired orientation. Stretch rate is determined by the chain speed, divergence an-
gle, and extent of orientation. The extent of orientation is determined by the
ratio of the width of the film entering to the width of the film leaving the sys-
tem. Tenter systems have been developed primarily to impart TD orientation.
Specialized systems allow simultaneous MD and TD stretching, but because of
the mechanical complexity of these systems usually only TD orientation is car-
ried out. The tenter frame, as part of a larger, sequential, biaxial-orientation sys-
tem, can be located before or after MD orientation; it is usually located after MD
orientation.

Depending upon polymer, the resin is extruded or coextruded as a flat sheet,

approximately 0.75–2.5 mm thick, onto a large chill roll. For PP, the chill roll is
in a water quench bath. The cast sheet is reheated and stretched in the longi-
tudinal direction on a series of heated chrome rolls and fed into a tenter chain

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FILMS, MANUFACTURE

297

Fig. 11.

Double-bubble orientation.

equipped with clips to grip the film. The chain system is located within a heat-
transfer oven, where the sheet is heated to the temperature at which it is to
be stretched. Heating is critical because the reliability of the stretching process
and the film properties depend upon the temperature, stretch rate, and stretch
ratio.

After preheating, the film is drawn in the transverse or cross direction by

the divergence of the tenter chains, annealed, and released. The edges of the film
are slit off, ground, and recycled, and the web may be wound full width or split
into narrower webs, which are electric discharge or flame treated for enhancing
printability and then wound onto rolls for further processing.

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FILMS, MANUFACTURE

Vol. 6

Fig. 12.

Melt orientation.

In contrast to the double-bubble process, the tenter-frame process permits

the direct manufacture of a film with improved dimensional stability because it
is heat-set by adding a length of chain after the divergent section at the finished
film width. This is very important for applications requiring good registration,
as in printing or photographic applications. The film can be heated or cooled to a
specific temperature while being restrained by the chain system. With line speeds
in excess of 300 m/min possible and widths of 8–10 m common, oven heat transfer
rates must also be high to control film annealing and crystallization.

Recently a new simultaneous biaxial orientation system has been developed

using linear motors to drive the clips and impart MD stretch as the film travels

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FILMS, MANUFACTURE

299

down the diverging tenter rails (37). This has replaced the mechanical complex-
ity of the earlier simultaneous systems with electronic control complexity, which
is readily handled by computer. This new tenter design has been implemented
commercially and offers the potential to develop films with new orientation ratios
and low melting surface polymers not possible with existing sequential stretching
systems.

Blown-Film Processes.

Bubble orientation technology permits simultane-

ous biaxial orientation directly from the melt (24) or by inflation of a quenched
tube. With homogeneous films, the latter method may give films of slightly differ-
ent properties than those of films produced on tenter frames. Melt orientation is
less effective and properties are different because of lower orientation.

Double-Bubble Method. In the double-bubble method (Fig. 11), the resin is

extruded as a tube that is quenched and gauged in cold water. The tube is then
reheated and oriented by blowing it into a bubble. Simultaneously, the speed of
the takeoff roll is increased for MD orientation; radiant electrical heaters con-
trol temperature. The bubble is collapsed, passed through nip rolls, reinflated,
annealed in a controlled-temperature chamber, and collapsed again. After trim-
ming the edges, it is separated into two webs, which are then treated by electric
discharge for printability and wound onto rolls.

Melt Orientation. In melt orientation, the polymer melt is extruded from an

annular die into a tube, which is simultaneously pulled away from the die as air
is blown into the tube and around the outer surface (by an air ring attached to the
die) to stabilize and quench the tube (24). This cools the melt as it accelerates in
the machine and in the radial direction, thus conferring orientation. The action
of the air solidifies the melt. The solidification point is called the frost line. The
frost line height is important in determining the reliability of the process and
the film properties. Above the frost line, the inflated tube is gradually collapsed
and fed to the haul-off nip, which supplies the power to pull the melt from the
die and prevents the bubble from collapsing by trapping air in the tube. The MD
orientation is determined by the speed at which the melt is pulled away from the
die (haul-off rate), the quenching rate (frost-line height), the die gap, and the melt
properties. The TD orientation is determined by the final diameter of the inflated
tube in relationship to the die diameter (blow-up ratio). The ratio of the die gap
to the film thickness is called the draw down ratio. It affects the stability of the
process and the orientation of the product. The tension pulling the melt away from
the die (24) is called the haul-off tension. It cannot exceed the melt strength or the
critical stress for melt fracture; the haul-off tension affects process stability and
film properties.

Tensilizing.

After biaxial orientation, film properties can be enhanced by

additional MD orientation. This step is called tensilizing, and is performed by MD-
drawing of the film after the initial biaxial MD and TD orientation, using sets of
rollers, as previously discussed. Tensilized film can be produced with improved
MD tensile strengths to permit use of thinner films as for magnetic recording
tape or to increase the MD shrinkage of the film for use as a shrink label for
complex packages such as cans and bottles.

A typical tensilized process consists of the casting step, the MD-orientation

step, the TD-orientation step, and another MD-orientation or tensilizing step.
This second MD orientation may be performed in-line as an additional step on the

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FILMS, MANUFACTURE

Vol. 6

orienter (38) or out-of-line as the film is slit for sale. It is also possible to add this
second MD orientation to a tenter-frame or double-bubble process (39).

Solvent-Solution Casting

Films and sheets may be made from polymers that cannot be melt-extruded or
that are extremely heat-sensitive by using polymer solutions. In this technology, a
homogeneous polymer solution is filtered and degassed, followed by formation and
drying of a gel film. The gel films are formed from the solutions by evaporation and
coagulation. Drying of the gel films into self-supporting films is the rate-limiting
step because it is diffusion-rate controlled. This also is of prime consideration in
the formation of thick films, where drying times may be excessive. Consequently,
these methods are generally limited to thin films. The characteristics of these films
are low residual orientation, uniform properties in all directions, and excellent
surface finishes (14,15).

Coagulation.

Gel formation by coagulation depends upon the differential

solubility of polymers in a blend of solvent, and nonsolvent, and solubility changes
resulting from concentration changes of added chemicals. For example, solvents
A and B are a good solvent and poor or nonsolvent, respectively, for polymer Y. A
solution of Y is prepared in A and extruded into a bath of solvent mixture of A and
B. The concentration difference causes diffusion of A out of the solution into the
bath and the diffusion of B into the polymer solution. As the concentration of B
increases and that of A decreases, the polymer becomes less soluble and eventually
precipitates as insoluble gel; complete evaporation of A and B leaves the finished
film. As an alternative, changes of pH by chemical additives may be sufficient
to reverse the solubility or change the chemical nature of a polymer in solution,
transforming it into an insoluble, stable gel. A good example of this technology is
the casting of cellophane (15) (Fig. 13).

Evaporation.

In this method (Fig. 14), the polymer solution is pumped

through a die, cast upon a stainless-steel belt, and transferred to a drying cham-
ber to evaporate. The solvent is recovered (15). As the solvent evaporates, a gel
forms. Further drying gives a stable film. The film is stripped from the belt and
rolled up.

Fig. 13.

Cellophane casting process.

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FILMS, MANUFACTURE

301

Fig. 14.

Solvent casting process.

Rolling

Rolling produces a film with predominately MD orientation by accelerating the
film from a nip point where the thickness is reduced. Large forces are found at
the nip point, but overall orientation can be increased over other forms of MD
orientation. Plastic strapping is oriented by this method (40).

Calendering

Calendering produces an unoriented cast sheet with uniform gauge at high
throughput (41,42). The equipment (Fig. 15) is expensive and consists of a stack
of especially hardened, driven rolls. The rolls are often supported in such a man-
ner that they may be bent or skewed relative to one another during operation.
This operation is required to maintain uniform thickness by overcoming the roll
bending forces developed in the calender nip. The high separation forces, ie 180–
1080 kg/cm of roll face, are generated by the melt (stock) viscosity and elasticity
as the material passes through the nip point.

Calenders are usually composed of four rolls that form three nips: feed nip,

metering nip, and finishing nip. The polymer is supplied to the feed nip from a
compounding operation and further mixed and heated by the circulating melt bank
formed at the feed nip. Alternatively, a melt of partially fluxed compound may be
supplied to the calender from an extruder or Banbury. The sheet is reduced at the
metering nip to the final desired thickness. The gauge uniformity is adjusted at
the finishing nip by bending the last roll or skewing the middle or third roll.

Finishing Operations

Coating.

Films are coated by metering a coating solution onto the surface

and removing the excess solvent in an evaporation oven. Hundred percent solid

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FILMS, MANUFACTURE

Vol. 6

Fig. 15.

Calendering-roll configurations: (a) three-roll superimposed type; (b) four-roll

inverted-L type; (c) four-roll Z type.

Fig. 16.

(a) Direct and (b) indirect gravure systems.

(solvent-less) coatings are becoming more popular and may be heat or radiation
cured. For metering, air knife-coaters, Meyer rod coaters, slot dies, and direct or
indirect gravure coaters (Fig. 16) are used. The film is unwound onto the coater and
may require surface treatment to ensure proper wet-out, especially with water-
based coatings. The film is coated at the coating station and transferred to a drying
oven. After drying, the film is rewound and ready for further processing (see also
C

OATING

M

ETHODS

).

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FILMS, MANUFACTURE

303

Embossing.

Embossing is used to modify the film texture or to decorate

the film with a three-dimensional pattern (14). The film is heated to the softening
point and distorted by passing it between a cooled roll with the three-dimensional
pattern and a nip. This immobilizes the distorted pattern imposed by the em-
bossing roll. The degree of distortion depends upon the film properties and the
processing conditions. Microembossing may also be performed on coextruded or
coated films to produce diffraction patterns and holographic images when metal-
lized. In this process only the surface is embossed by first heating the film or just
the film surface and then passing it through a nip between the etched embossing
roll and a backing roll to force the polymer surface into the etched roll. This force is
required to transfer the etched roll pattern to the film surface. The pattern is then
enhanced by metallization of the film to cause light refraction from the patterned
surface. This is a growing use for films in decorative and security applications.

Metallization.

Film metallizing is a growing use for films for both decora-

tive and the functional enhancement of light and gas barrier properties. A thin
layer of aluminum metal coats film by a process of physical vapor deposition or
sputtering. Rolls of film are mounted into a high vacuum chamber and evacuated
to approximately 1

× 10

− 5

torr. Once this pressure level is reached aluminum

metal is melted and flash evaporated on ceramic boats. The film is passed above
the evaporation area at speeds approaching 500 m/min. This deposits a layer
of aluminum approximately 100 nm thick. This layer thickness of aluminum re-
flects approximately 99% of the incident light and reduces the amount of moisture
and oxygen permeation through the film to create an excellent packaging film for
products subject to rancidity. Industrial uses for static dissipation, visible and IR
radiation reflective window films, as well as clear IR radiation reflective films for
glazing have also been widely used.

Surface Treatment.

Most film surfaces require surface treatment for use

in subsequent steps of coating, printing, lamination, or metallization. This treat-
ment results in the chemical modification of the polymer at the surface. The most
widely employed surface modification is oxidation of the polymer to create a polar
surface (43). Processes that are widely employed are corona and flame treatment.
Wet chemical oxidative treatments are seldom used and are restricted to low speed
operations. A new surface oxidation technique has been recently developed which
uses an atmospheric plasma generated in a hollow cathode with helium or helium
gas blends (44).

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6. H. Swada, in K. F. O’Driscoll, ed., Thermodynamics of Polymerization, Marcel Dekker,

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25. U.S. Pat. 4,173,417 (Nov. 6, 1979), G. A. Kruder (to HPM Corp.).
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43. C.-M. Chan, Polymer Surface Modification and Characterization, Hanser/Gardner,

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44. U.S. Pat. 6,118,218 (Sept. 12, 2000), A. Yializis, S. A. Prizada, and W. Decker (to Sigma

Technologies International, Inc.).

E

LDRIDGE

M. M

OUNT

III

EMMOUNT Technologies

FILMS, MULTILAYER.

See C

OEXTRUSION

.

FILMS, ORIENTATION.

See Volume 2.